A fixture for dental illumination, in which an elliptical reflector and light source is used with a light gate and projection lens, these being set in an enclosing canister with the light emerging through the lens, an infrared filter in the path of the light rays to absorb unwanted heat radiation, and...http://www.google.com/patents/US3704928?utm_source=gb-gplus-sharePatent US3704928 - Dental light with dichroic and infrared filters

A fixture for dental illumination, in which an elliptical reflector and light source is used with a light gate and projection lens, these being set in an enclosing canister with the light emerging through the lens, an infrared filter in the path of the light rays to absorb unwanted heat radiation, and a dichroic filter on the surface of the lens facing the reflector to correct the light-rendering index of the emitted light. Without any filter, neither infrared nor visible, the color rendering index of the light from the device is 97.5. When the infrared filter is added, this drops to 88.0, but the addition of the dichroic visible-light filter brings this back to 97.5, which is very good.

350/316; 240/4115 413; 353,55 infrared filter in the path of the light rays to absorb unwanted heat radiation, and a dichroic filter on the [56] References Cited surface of the lens facing the reflector to correct the UNITED STATES PATENTS light-rendering index of the emitted light. Without any filter, neither infrared nor visible, the color rendering 3,371,202 2/1968 Moore et a1. ..240/41.3 index of the light from the cievice is 97 5 w the 2,798,943 7/1957 Pndeaux ..353/55 X infrared filter iS added, this drops to 0 but the 2,827,554 3/1958 Gunther et a1 ..350/316 X dition of the (chi-oic visih|e iight filter brings ihiS 3,113,033 12/1963 Hoxie et al. ..350/1 X back to 97 which is very good.

2. Brief Summary of Prior Art Dental fixtures have been developed using a light source at one focus of an ellipsoidal reflector, and at the other focus a gate having an opening defining the extent of the beam. These provide a high intensity of light on the patients face, but since the light source is an incandescent lamp, most of its radiated energy is infrared, which does not assist the dentist in seeing, but can undesirably heat up the face of the patient on which it falls. A filter plate is used in front of the lens, that is between the lens and the patient's face to absorb the infrared and produce a cooler light.

The usual infrared filters have a broad cutoff and also absorb visible light in some parts of the spectrum. The quality of the light emitted from the fixture without the filter is such that its color-rendering index is 97.5, which is considered excellent, but the addition of the filter drops this index to 88, which is undesirable, since the dentist needs to see the patients teeth in their natural colors.

The glass filters available for cutting off the far infrared are found to transmit more in the green and yellow than in the blue, and since the incandescent source is comparatively low in blue radiation, this means that the green light will predominate and give the light a greenish tinge which is undesirable.

BRIEF SUMMARY OF INVENTION It is here that our invention solves the problem by using an additional filter which cuts down on the green and has high transmission for the blue. The filter also cuts down on the yellow, orange and red somewhat, but since the source is rich in these colors anyway, this merely helps to give a balanced light. The color temperature is raised to an acceptable balance, and the color rendering index is brought back to 97.5. We have discovered that this filter can best be utilized by making it of the dichroic type, which can be coated onto some light-transmitting part of the device. The dichroic filter was tried at the light gate, but the high radiation density at that point destroyed the dichroic coating. A coating on the back of the infrared filter, that is, on the side nearest the lens, was found to give secondary reflections back onto the gate, which was transmitted into the illuminating pattern, which was undesirable. A third approach was tried by coating the external surface of the infrared filter, but this left the coating vunerable to abrasion when cleaned.

We have discovered that all these disadvantages can be overcome by having a dichroic coating on the lens surface which faces the source of light. In the optical system used, this was the spheric side of the lens, the other side being aspheric.

BRIEF SUMMARY OF DRAWINGS Other features, objects and advantages of the invention will be apparent from the following specification, taken in connection with the accompanying drawings in which:

FIG. 1 is a graph of transmittance against wavelength for the infrared filter and for the dichroie filter on the lens; and

FIG. 2 is a longitudinal section through the centerline of the fixture.

DESCRIPTION OF SPECIFIC EMBODIMENTS In FIG. I, the dotted curve shows the transmission of the infrared filter for visible light and is to be somewhat low in the blue (400-500 millimicrons), higher in the green and yellow (500-600 millimicrons), and low again in the red (600-750 millimicrons). This, in conjunction with the incandescent filament source, produces a somewhat greenish light.

The full-line curve shows the transmission of the diehroie filter, which is high in the blue, and low in the yellow and green, rising somewhat toward the red.

Although only the visible transmissions of the filters are shown, the transmission of the infrared filter drops to zero at about 900 millimicrons and continues there up to at least 2,700 millimicrons, which was the limit of the measuring equipment.

Referring to FIG. 2, a cross sectional view of the medical and dental fixture is shown. The major components comprise a main housing canister I0, an optical system I2 located within the canister and a flexible boot l4 affixed to the rear end of the canister. The boot 14 covers and holds the electrical connections for a lamp [6. The lamp, being one of the components of the optical system of the fixture, is positioned within an ellipsoidal-shaped reflector I8. The lamp and reflector produce an intense source whose light is directed through a rectangular opening 24 centrally located in a fixed light gate 20. A spacing rim 22 spaces the lamp and reflector from the light gate so that an optimum amount of light produced by the lamp 16 passes through the gate opening 24 and thence to an aspheric lens 26, and an exterior heat absorbing cover glass 32.

The lens 26 and heat absorbing cover glass 32 are spaced from one another and held by a circular bezel 30. This segment of the optical system produces a very sharply defined rectangular light pattern.

The optically clad glass rod 40 shown in FIG. 2 is made of a glass rod probe with optically polished ends and encased within a metal casing. One end of the rod 40 extends into the housing 10, and passes through the reflector and terminates just short of the envelope 51 of the lamp, and perpendicular to the axis of the filament 50. The other end of the rod 40 extends outwardly from the housing 10 and is connected to a flexible fiber optic conduit comprised ofa standard bundle of flexible glass fibers 44 in the shape of a conduit having an approximately circular cross section.

With this arrangement the rod 40 collects from the filament light which travels over the conduit 44 to emerge at the light exit end 48.

The lens surface facing the source of light, that is, the back surface 28 of the lens 26 is coated with a dichroic filter 9 which transmits more blue light than green, and

which also passes the red. The filter can be made in the usual manner with successive thin coats or layers of magnesium fluoride and zinc sulfide, each about onefourth wavelength thick, there being 9 coats of magnesium fluoride and 10 coats of zinc sulfide in all. The coatings can be applied by vacuum disposition in the usual manner.

What we claim is: l. A dental lighting fixture comprising: an optical system including an ellipsoidal reflector having a light source disposed at one focus thereof; a fixed light gate disposed in front of said optical system, said gate having a rectangular opening disposed at the other focus of said ellipsoidal reflector;

a lens disposed in front of said light gate, said lens having a spheric side facing said light gate, the other side of said lens being aspheric;

a dichroic coating disposed on said spheric side of said lens, said coating having higher transmission of blue light than of green, yellow, orange or red light; and

an infrared absorbing glass fllter disposed in front of said lens, the combination of said dichroic coating and said filter yielding light of high color rendering index.